The lifespans of porcupines in a particular zoo are normally distributed. The average porcupine lives $20.2$ years; the standard deviation is $4.7$ years. Use the empirical rule (68-95-99.7%) to estimate the probability of a porcupine living longer than $29.6$ years.
Answer: $20.2$ $15.5$ $24.9$ $10.8$ $29.6$ $6.1$ $34.3$ $95\%$ $2.5\%$ $2.5\%$ We know the lifespans are normally distributed with an average lifespan of $20.2$ years. We know the standard deviation is $4.7$ years, so one standard deviation below the mean is $15.5$ years and one standard deviation above the mean is $24.9$ years. Two standard deviations below the mean is $10.8$ years and two standard deviations above the mean is $29.6$ years. Three standard deviations below the mean is $6.1$ years and three standard deviations above the mean is $34.3$ years. We are interested in the probability of a porcupine living longer than $29.6$ years. The empirical rule (or the 68-95-99.7 rule) tells us that $95\%$ of the porcupines will have lifespans within 2 standard deviations of the average lifespan. The remaining $5\%$ of the porcupines will have lifespans that fall outside the shaded area. Because the normal distribution is symmetrical, half $({2.5\%})$ will live less than $10.8$ years and the other half $({2.5\%})$ will live longer than $29.6$ years. The probability of a particular porcupine living longer than $29.6$ years is ${2.5\%}$.